The blowing or stretch blowing molds used for producing containers from a preform made of thermoplastic material which is preheated generally consist of three parts, for the particular shape of current containers at their base, which is generally concave. Therefore, generally, molds are made which consist of two lateral half molds which shape the body of the container and of a mold bottom to shape the container bottom. Therefore, the lower areas of the two half molds and the upper part of the mold bottom are provided with fixing means, of complementary shape, reciprocally insertable into each other when the mold is in the closed position, to ensure the axial rigidity of the mold in closed position in the presence of the blowing pressure, which generally reaches 40 bar when molding PET containers. The two lateral half molds are configured to move relative to each other between an open position and a closed position, under the action of electrically controlled actuating means or via cams. The same actuator means are also arranged to control the cyclic opening and closing of the mold bottom. Solutions are particularly widespread which provide for the opening and closing of the two half molds and of the mold bottom by means of cams. Usually, the maximum angle to be formed upon reaching the maximum opening of the two lateral half molds is closely related to the maximum size of the cross section of the containers to be extracted from the two half molds after molding. This therefore has an influence on the linkage and on the geometry of the cam profile. It is known that, in the case of container molding devices, the maximum productivity is measured in containers/h/cavity and is expressed as a function of cycle time, and said cycle time is made up of a step dedicated to the insertion of a preform into the mold and the closing and constraint movement of the mold, and of a step dedicated to the blow molding process, and thus the cycle time also includes a step dedicated to the release, i.e. the opening of the half molds and the extraction of the finished container from the blowing mold. Therefore, the amplitude of the maximum opening angle of the mold has an influence on the duration of the cycle time. In designing the blow molding machines of thermoplastic containers, the maximum size of the containers which can be produced in such machines and the maximum forces which are applied on the various parts of the machine, and in particular the reactions which are transmitted to the cams by the action of the actuator means during the operations, are normally defined. When containers of a certain size are to be produced, the mold is designed to open by a first maximum opening angle α1. If containers of smaller sizes are also to be produced on blow molding machines designed to produce containers of a larger format, and thus to open by the first maximum opening angle α1, a maximum opening angle of the mold α2 smaller than α1 would be sufficient for the extraction of the smaller container from the mold. Disadvantageously, if such smaller bottles are to be produced, since the mold is opened by the maximum design opening angle, i.e. α1, which is associated with larger bottle sizes, the maximum theoretical productivity achievable by the blow molding machine is reduced because of the longer time taken to open and close the half molds by the aforementioned angle α1. This is a strong limit to the increase of the productivity of blow molding machines. Solutions have been proposed to try to achieve higher productivity levels for blow molding machines, according to which the molds are designed with a single value of the maximum opening angle which is reduced and hence they are able to produce smaller types of containers. For example, there are molding devices optimized to produce containers up to 0.5 liters max, or up to 2.0 liters max, etc. However, in these cases, the blow molding machines put the achievement of high productivity first, but on the other hand they lose flexibility in the production capacity of containers of different sizes.
In fact, it is not possible to produce, on the same device, containers having dimensions exceeding the limits imposed by the spaces available for the extraction of the same. In blow molding machines of this type, in order to change the maximum opening angle between the two half molds, it is necessary to intervene on the linkage members, for example by changing the size of some elements of the transmission, for example the connecting rods. However, this operation is complicated because it requires a long time both for installation and for the subsequent adjustments of the mechanical members and it is disadvantageous because it consequently forces a long interruption of production. An example of systems for the control and the cyclic closing of the mold in PET bottles which aims to improve the productivity of a blow molding machine is described in document EP 2135726, which provides for the two lateral half molds to open and close in an unequal manner to the plane of vertical symmetry of the mold. In particular, the half mold which is in retracted position, considering the direction of motion of the blow molding machine, has an opening speed which reaches its maximum value in a step prior to the step in which the half mold which is ahead reaches its maximum value. Therefore, the two maximum opening angles, which reach the two half molds when they are fully open, are of equal value but the moments in which the half molds reach these maximum angles are consecutive, i.e. not concurrent. Another example of a control system for opening and closing a blowing mold in a rotary blow molding machine is disclosed in document U.S. Pat. No. 7,871,259. In order to improve the opening and closing kinematics of the half molds and reduce the distance which separates adjacent blowing molds, this document proposes a configuration of the control system for opening and closing a blowing mold with the plane of separation of the half molds directed obliquely, in the direction of advance of the blow molding machine, by a selected angle in relation to the radial direction of the blow molding machine, and with one of the two half molds rigidly connected to the blowing wheel (not movable). A drawback of this solution is that it can pose problems of extraction of the bottle from the mold, because the bottle can remain adherent to the non-movable half mold during the extraction step by the gripping members. Therefore, the need is felt to provide a device for opening and closing the mold which offers greater flexibility in the change of the formats of the containers to be blown on the same blow molding machine.